1. Field of the Invention
This invention relates to edible food products produced by partial deamidation of oilseed proteins.
2. Description of the Prior Art
Oilseeds are a good source of low-cost protein, but the food value of the protein can often be improved by chemical or enzymatic modification. Enzymatic treatments have been reported to improve the solubility of oilseed proteins but sometimes destroy their emulsifying capacity and foam stability. Chemical modifications, using techniques which include sulfonation, succinylation, phosphorylation, and deamidation, invariably improve functionality of the oilseed proteins.
Of the chemical methods mentioned above, the hydrolysis of amide groups (deamidation) is particularly attractive because oilseed proteins have an unusually large number of such groups. Significant improvements in functionality can be achieved even at low levels of deamidation. Deamidation facilitates dissolution of the protein in water because more polar-charged glutamic and aspartic acid groups replace the less polar glutamine and asparagine groups during deamidation. The enhanced solubility enables the protein to better exert its surfactancy in lowering the surface tension between water and oil (emulsion) or water and air (foam) at their interface. The deamidated protein therefore becomes more valuable for food use because it is more soluble, more whippable, and has a greater ability to emulsify oil.
The extent of hydrolysis required to produce significant deamidation, however, requires the hydrolysis to be conducted under rather severe conditions involving temperatures of 100.degree. C. or higher and strong acids such as HCl at concentrations of 0.5N or higher. Under these conditions, considerable degradation of the protein molecule occurs which causes undesirable changes in the protein such as bitter taste and reduced functionality. Specifically, while it is desirable to achieve a protein deamidation of 30% or higher for significant improvements in functionality, the concomitant peptide bond hydrolysis, at 6% or higher under normal deamidation conditions, will most likely have a major effect (often detrimental) on the functional properties of the treated protein.
Forty-five years ago, J. Steinhardt and C. H. Fugitt investigated the effect of various anions on protein hydrolysis, using wool as the water insoluble protein substrate and egg albumin as the water soluble protein substrate (J. Res. Natl. Bur. Standards 29, 315 (1942)). They reported that these proteins were hydrolyzed at the amide and peptide bonds more effectively in the presence of some long carbon-chain anions including alkylsulfates and alkanesulfonates. However, the study was not intended for practical applications and there was no recording of property changes which occurred to the wool and egg albumin from the above mentioned catalytic hydrolysis. Other investigations concern the uses of ion exchange resin as a catalyst for protein hydrolysis, J. C. Paulson et al., J. Am. Chem. Soc. 75, 2039 (1953); J. R. Whitaker and F. H. Deatherage, J. Am. Chem. Soc. 77, 3360 (1955). However, these latter studies are interested only in total protein hydrolysis and fail to distinguish between deamidation and peptide bond hydrolysis.